A conventional exhaust pipe for a vehicle-mounted engine in which a muffler and an exhaust gas purification catalyst are disposed adjacent to each other is described, for example in Patent Document 1. FIG. 1 also illustrates such conventional exhaust pipe for a vehicle-mounted engine.
As illustrated in FIG. 1, an exhaust pipe 1 includes an outer pipe 10 and an inner pipe 31. An exhaust gas purification catalyst 21, which is provided in an exhaust system of a vehicle-mounted engine to purify exhaust gas, contacts the outer pipe 10 inside the outer pipe 10. An inner pipe 31 is provided within the outer pipe 10 to be located adjacent to the exhaust gas purification catalyst 21 on the downstream side of exhaust gas with respect to the exhaust gas purification catalyst 21. The inner pipe 31 includes a separator 32 having a tapered portion 32A, a diameter of which is reduced toward the downstream side of exhaust gas, and a linear inside pipe 33 connected to the separator 32 on the downstream side of exhaust gas. An end 32C of the separator 32 located on upstream side of exhaust gas is partly and fixedly attached to the inner peripheral surface of the outer pipe 10, for example by spot welding, in the circumferential direction of the end 32C. A part of the inside pipe 33 located on the downstream side of exhaust gas contacts the end 13 of the outer pipe 10 on the downstream side of exhaust gas inside the outer pipe 10 via a wire mesh 34 to absorb the difference in heat expansions between the inside pipe 33 and the outer pipe 10. This allows the inside pipe 33 to move relative to the outer pipe 10 in the axial direction of the inside pipe 33. The outer peripheral surface of the inner pipe 31 (i.e., the separator 32 and the inside pipe 33) and the corresponding inner peripheral surface of the outer pipe 1 define an expansion chamber 35. The temperature and pressure of exhaust gas introduced into the inside pipe 33 via the separator 32 are decreased when exhaust gas flows out to the expansion chamber 35 through a plurality of communication holes 33A that are formed in the inside pipe 33. Thus, in the exhaust pipe 1, the outer pipe 10 and the inner exhaust gas purification catalyst 21 within the outer pipe 10 serve as a catalyst device 2 and the outer pipe 10 and the inner pipe 31 serve as a muffler 3.
In the exhaust pipe 1, as described above, the catalyst device 2 and the muffler 3 are integrally formed of the same outer pipe 10. This enables the reduction in steps of manufacture and reduction in the number of parts, compared with the exhaust pipe in which a catalyst device and a muffler are formed of different outer pipes. In addition, the entire length of the exhaust pipe 1 can be shortened, which improves the mounting of the exhaust pipe 1 in a vehicle.
In the muffler 3 of the exhaust pipe 1, exhaust gas introduced into the inside pipe 33 via the separator 32 is flown out to the expansion chamber 35 through the communication holes 33A. In this situation, water vapor contained in the exhaust gas is cooled down by contacting the inner peripheral surface of the outer pipe 10 to turn into condensed water to be accumulated in the expansion chamber 35. Meanwhile, since the separator 32 and the outer pipe 10 are only partly attached fixedly in their circumferential directions, there is a gap between them. Thus, it may occur that a part of condensed water in the expansion chamber 35 flows out toward exhaust gas purification catalyst 21 between the separator 32 and the outer pipe 10 to contact the exhaust gas purification catalyst 21. If the condensed water contacts the exhaust gas purification catalyst 21, the temperature of the exhaust gas purification catalyst 21 would rapidly decrease to generate heat shock or the exhaust gas purification catalyst 21 would be chemically changed, which may result in deterioration of purification function of the exhaust gas purification catalyst 21. This tendency becomes prominent not only in the configuration where the outer pipe and the end of the inner pipe that contacts the inner peripheral surface of the outer pipe are partly and fixedly attached in their circumferential directions but also in the configuration where the end of the inner pipe contacts the outer pipe in the outer pipe via a wire mesh.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-132320